Aside from the aforementioned possible issue of mixing RAM, it's worth noting that these days, every motherboard and their dog claims to support 8GB or higher. Unfortunately, in practice many have difficulty powering all 4 memory slots at once, and require a slight boost in voltage in order to stay properly stable. As long as you have half-decent airflow in your case, an extra 0.1 or 0.2 volts to your memory shouldn't be an issue for those sticks.